This invention relates to a probe used to carry out nuclear magnetic resonance (NMR) measurements on a sample under pressure. Pressure vessels are used to contain the sample when a high pressure environment is needed to conduct the experiment or produce the desired compound to be subjected to NMR. If high pressure is not a requirement, an ambient pressure probe is used. To the best of the applicant's knowledge, all commercially produced probes have fallen into this latter category while the high pressure probes have been specifically designed and constructed according to an experimenter's specifications on an individual basis.
NMR is a useful tool for determining molecular structure through the examination of the response of nuclear magnetic moments to an externally applied homogeneous magnetic field (B0). The probe used in high pressure NMR houses the sample and is placed in the B0 field in a specified orientation with respect to the magnetic field lines. The presence of B0 causes the magnetic moments of the targeted class of nuclei in the sample to process about the field's axis at a rate which is dependent on the magnetic field strength. A transmitter coil internal to the vessel and oriented to apply a magnetic field (B1) perpendicular to B0 is then energized at a specified RF frequency termed the Larmor frequency. The newly created magnetic field, B1, interacts with the nuclear magnetic moments processing about the BO axis. This interaction of the B1 field with the nuclear magnetic moments shifts the precession of the magnetic moments to the B1 axis during the period the RF field is energized. The RF field is then deenergized and the sample response to the previously applied RF field is received using the same coil as was used for the RF transmission. Due to the dual function of the coil, the transmission and reception phases alternate over a specific period. The received signal from the energized nuclei serves as the input signal for spectroscopic analysis of the sample.
The sensitivity of the coil during its receive phase is dependent on the coil efficiency which, absent the effect of the materials used in construction, is highly dependent on the shape of the coil and its distance from an electrically conducting body. Three coil configurations are generally considered for use as the RF coil: the Helmholtz, the solenoidal, and the toroidal. Of the three, the toroid is the most efficient for a metal pressure vessel. This is due to the greatly reduced coil-vessel magnetic coupling associated with the toroid which results from the confinement of the magnetic field, produced by the RF transmission, to the volume enclosed by the toroid. The other coil configurations, the Helmholtz and the solenoidal, do not produce a confined field; rather, they produce a field both interior and exterior to the confines of the coil. As a result, these coils incur a significant magnetic coupling effect with the metal container walls unless placed sufficiently far from the conducting body. The presence of a coil-vessel magnetic coupling results in an energy loss to the conductor in the form of eddy currents during transmission phase of the RF signal. During the receive phase, this coupling causes the coil to pick up thermal and other noises from the conducting walls of the vessel housing the sample and coil. This results in a smaller absolute signal to noise ratio (S/N) for the Helmholtz and solenoidal coils when compared to the absolute signal to noise ratio of the toroid; as a result the effectiveness of the prior coils in receiving the signal from the sample is compromised when compared to that of the toroid. Since, as indicated by Glass and Dorn, "BI and B0 Homogeneity Considerations for a Toroid-Shaped Sample and Detector", Journal of Maonetic Resonance, vol. 51, 1983, pp. 527-530, the majority of coils used in the detection of NMR signals for superconducting magnetic systems are of the Helmholtz type, the poor signal to noise ratio is a problem in accurate data acquisition. To help alleviate the magnetic coupling problem associated with coil generated fields and the surrounding conducting bodies, it was recommended by Hoult that coils which develop this coupling should be positioned a distance equal to the coil's largest dimension from the nearby conductors. This greatly increased the required size of the pressure vessel or other conducting container, and resulted in magnetic field requirements which were more difficult to achieve due to increased size requirements. For a pressure vessel this essentially increases the inside diameter of the vessel by a factor of three with a resulting increase in the required wall thickness. Since the use of a toroid essentially eliminates magnetic coupling with the wall, these spacing requirements are essentially eliminated.
Glass and Dorn, "A High Sensitivity Toroid Detector for O NMR", Journal of Maonetic Resonance. vol. 52, 1983, pp. 518-522, experimented with a toroid formed on a Pyrex glass cell as a possible alternative to the standard Helmholtz coil in an effort to produce an improved S/N ratio. However, to obtain this improved S/N, they determined that the ratio of the inner to outer radius (R2/R1) should be less than 1.4 so as not to compromise the B1 field homogeneity. However, this radius limitation requirement results in a smaller volume within which to house the sample. Thus, in an effort to retain the volume needed to contain the sample while at the same time keeping the R2/RI low, Glass and Dorn suggested elongating the toroid in a direction perpendicular to the plane of the toroid. With this reconfiguration of their glass cell, they obtained a slight improvement in the S/N when compared to a circular cross section toroid with the same R2/R1. However, they also determined that their toroid degraded the spatial homogeneity of the B0 field above the toroid causing a resulting degradation in the achievable resolution. As a result, a reduction in the BO spatial homogeneity places a limit on the type nuclei the probe can be used to analyze. To counter this, they had to design specific shim coils for placement above and below the torus region. This action resulted in an improvement in the B0 homogeneity; however, the requirement for specialized shim coils leads to much greater operational complexity particularly when a pressure vessel is required to contain the sample. They attempted to use a standard commercially produced shim which is employed external to the probe in the spectrometer, but they were not able to duplicate the results of their specifically designed shim.
Although the toroid would appear to offer many advantages over other coil configurations, it also produced several problems in the areas of B0 field homogeneity, R2/RI relationships to B1 homogeneity, the necessity for specifically designed shims and sample resolution. Therefore, one of the objects of the present invention is to improve on the capability of the toroid to function as an RF coil in a NMR probe.
Another object of the present invention is to provide a high pressure vessel capable of housing the pressurized sample and the toroid coil in a manner compatible with NMR spectroscopy.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.